What agitates me somewhat, reading this, is my BS meter keeps getting a signal.
First, the OP is being told by the dealer that GM won't let them clean the valve.
Since it has been determined that the part and labor are out of warranty, GM has no input that must be followed by the dealer.
If the dealer wanted to clean it, they could have.....since GM wasn't paying anything they could have thrown it out in a field if they wanted to.
Since the OP had to pay for the part, a non-serviceable part with no core value, it is the OP's property and should have been returned.....or at least the OP should have been given the option if the old part was to be returned.
Chevrolet TAC may have suggested valve replacement, may have said it is not serviceable, but was never in a position to say 'It cannot be cleaned'
I find that response rather annoying.......'Can't' is a term that really means 'Won't'
Rob